As mobile devices have been increasingly developed and the demand for such mobile devices has increased, the demand for secondary batteries has sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and discharge voltage, into which much research has been carried out and which is now commercialized and widely used.
Based on the appearance thereof, the lithium secondary battery may be generally classified as a cylindrical battery, a prismatic battery, or a pouch-shaped battery. Based on the type of an electrolyte, the lithium secondary battery may be generally classified as a lithium ion battery or a lithium ion polymer battery.
A recent trend in the miniaturization of mobile devices has increased the demand for a prismatic battery or a pouch-shaped battery, which has a small thickness. An exploded perspective view of a pouch-shaped battery and a perspective view of an assembled state of the pouch-shaped battery are typically shown in FIGS. 1 and 2, respectively.
Referring to FIGS. 1 and 2, a pouch-shaped battery 10 includes a pouch-shaped case 20 having an inner space 21 of a predetermined size, a cover 30 hingedly connected to the pouch-shaped case 20, an electrode assembly 40 mounted in a receiving part 21 of the pouch-shaped case 20, the electrode assembly 40 including cathode plates 41, anode plates 42, and separators 43, electrode tabs 41a and 42a extending ends of the cathode plates 41 and the anode plates 42 of the electrode assembly 40, respectively, and electrode terminals 50 and 51 connected to the electrode tabs 41a and 42a, respectively.
A side extension part 22 of a predetermined width for thermal bonding is formed at an upper edge of the receiving part 21 of the pouch-shaped case 20. Middle portions of the electrode terminals 50 and 51 respectively connected to the electrode tabs 41a and 42a are coated with terminal tapes 52 made of an insulative material to prevent the occurrence of a short circuit between the electrode terminals 50 and 51 when the side extension part 22 of the pouch-shaped case 20 is thermally bonded to a side part 31 of the cover 30 using a thermal bonding device (not shown).
The conventional pouch-shaped battery with the above-stated construction is manufactured as follows.
First, the electrode assembly 40 including the cathode plates 41, the anode plates 42, and the separators 43 is mounted in the receiving part 21 of the pouch-shaped case 20 and then a predetermined amount of an electrolyte is injected into the inner space of the pouch-shaped case 20. At this time, the electrode tabs 41a and 42a of the electrode assembly 40 are respectively connected to the electrode terminals 50 and 51, the middle portions of which are coated with the terminal tapes 52. The electrode terminals 50 and 51 and the terminal tapes 52 partially protrude outward from the pouch-shaped case 20 and the cover 30.
Subsequently, the cover 30 is brought into tight contact with the pouch-shaped case 20 and then the side extension part 22 of the pouch-shaped case 20 is thermally bonded to the side part 31 of the cover 30 using the thermal bonding device (not shown) such that the electrolyte does not leak from the pouch-shaped case 20.
The shape of a representative battery pack having the pouch-shaped battery with the above-stated construction mounted therein is shown in FIG. 3 and a separated state of the battery pack before being assembled is typically shown in FIG. 4.
Referring to FIGS. 3 and 4, a battery pack 60 includes a rectangular battery 10 having an electrode assembly including cathodes, anodes, and separators received therein together with an electrolyte in a sealed state, a case body 70 having an inner space to receive the battery 10, and an upper cover 80 coupled to the case body 70, in which the battery 10 is received, to seal the battery 10. Between the case body 70 and the battery 10 and between the upper cover 80 and the battery 10 are attached double-sided adhesive tapes 90.
In general, the battery pack 60 with the above-stated construction is assembled by coupling the upper cover 80 to the case body 70, which is made of a plastic material, such as polycarbonate (PC) or acrylonitrile-butadiene-styrene (ABS), using an ultrasonic welding method. The ultrasonic welding method is a method of thermally bonding two surfaces using a frictional heat generated due to vibration based on a high frequency of 20,000 Hz.
However, the demand for a battery pack having a smaller thickness has increased. In recent years, therefore, the thickness of the case body 70 and the upper cover 80 has been reduced to 0.3 to 0.35 mm. As a result, it is difficult to perform die molding and injection molding. In addition, welding strength is reduced with the result that a welding defect rate is increased.
For a battery using a can as a battery case, on the other hand, it is possible to provide proper strength against external impact due to structural characteristics of the can even in a case in which the thickness of the battery case is small. However, the pouch-shaped battery 10 having the structure shown in FIG. 1 has low strength against external impact due to structural characteristics of the pouch-shaped battery 10. For this reason, application of a case having a small thickness to the pouch-shaped battery 10 is limited.
Furthermore, when external impact is applied to the battery pack 60, the battery 10 may move upward and downward in the inner space defined between the case body 70 and the upper cover 80 although the battery 10 is coupled to the case body 70 and the upper cover 80 using the double-sided adhesive tapes 90 with the result that a short circuit or a cut off may occur in the battery pack 60.
Specifically, when the pouch-shaped battery are located in an inner space of a pack case constituted by the case body 70 and the upper cover 80, an empty space is formed between the pouch-shaped battery and the inside of the upper end of the pack case due to electrode terminals, a protection circuit module (PCM), an insulating member, etc. mounted or coupled to the upper end of the pouch-shaped battery. The upper end of the battery is relatively weak. When the battery pack falls or external impact is applied to the battery pack, therefore, the battery may be easily deformed with the result that the battery may have defects. For example, when the battery moves toward the upper end of the inner space of the pack case due to falling of the battery pack or external impact applied to the battery pack, a short circuit may occur due to electric contact between the devices. On the other hand, when the battery moves toward the lower end of the inner space of the pack case, electrical connection between the devices located at the upper end of the battery may be cut off
Consequently, there is a high necessity for a battery pack that can be easily manufactured and has proper strength against external impact and high resistance to a short circuit or a cut off while using a case having a small thickness.